means An empty space or position waiting to be filled, especially a room available for rent or a job opening with no one in it.

from From the Latin 'vacare,' meaning 'to be empty' or 'to be free,' which also gives us 'vacant,' 'vacation,' and 'vacuum.' The word arrived in English by way of Latin's branch of meanings, all circling the same idea of unoccupied space. The motel sign sense — that glowing red word in the dark — is a much later, distinctly modern flourish, but the root has always pointed at the same thing: a place where something or someone simply isn't yet.